I’ve just come across an excellent post regarding why people should avoid using CAPTCHA. CAPTCHA is designed to prevent webmasters from having their sites bombarded with spam by introducing a test which must be past before a comment/contact form can be submitted. Unfortunately many of these captchas are visual and can not be read by blind people who use screen reading software (I.E. software which converts text into speech and braille enabling them to read a computer’s screen). There are (sometimes) audio alternatives to the visual captcha on which a visually impaired person may click, however there are still a lot of sites who do not use audio captchas and such captchas are of no practical use to a person who is both blind and profoundly deaf.
As mentioned in a previous post there is a plug-in, for Firefox called Webvisum which allows a visually impaired person to submit a CAPTCHA, have it solved and sent back to them. This is a good tool, however it is not 100 per cent effective. Only yesterday I attempted to register for a site using a purely visual captcha. Even after several attempts, using Webvisum the CAPTCHA remained unsolved.
For a good discussion of the reasons not to use CAPTCHA please see http://onecoolsitebloggingtips.com/2010/11/14/the-10-point-case-against-captcha-use/#comment-21053.